Metabolism of Fostamatinib, the Oral Methylene Phosphate Prodrug of the Spleen Tyrosine Kinase Inhibitor R406 in Humans: Contribution of Hepatic and Gut Bacterial Processes to the Overall Biotransformation

Abstract

ABSTRACT:The metabolism of the spleen tyrosine kinase inhibitor N4-(2,2-dimethyl-3-oxo-4-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethyoxyphenyl)-2,4-pyrimidinediamine (R406) and its oral prodrug N4-(2,2-dimethyl-4-[(dihydrogenphosphonoxy)methyl]-3-oxo-5-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethyoxyphenyl)-2,4-pyrimidinediamine disodium hexahydrate (R788, fostamatinib) was determined in vitro and in humans. R788 was rapidly converted to R406 by human intestinal microsomes, and only low levels of R788 were observed in plasma of human subjects after oral administration of 14 C-R788. R406 was the major drug-related compound in plasma from human subjects, and only low levels of metabolites were observed in plasma. The plasma metabolites of R406 were identified as a sulfate conjugate and glucuronide conjugate of the para-O-demethylated metabolite of R406 (R529) and a direct N-glucuronide conjugate of R406. Elimination of drug-related material into the urine accounted for 19% of the administered dose, and the major metabolite in urine from all the human subjects was the lactam N-glucuronide of R406. On average, 80% of the total drug was recovered in feces. Two drug-related peaks were observed; one peak was identified as R406, and the other peak was identified as a unique 3,5-benzene diol metabolite of R406. The 3,5-benzene diol metabolite appeared to result from the subsequent O-demethylations and dehydroxylation of R529 by anaerobic gut bacteria because only R529 was converted to this metabolite after the in vitro incubation with human fecal samples. These data indicate that the major fecal metabolite of R406 observed in humans is a product of a hepatic cytochrome P450-mediated O-demethylation and subsequent O-demethylations and dehydroxylation by gut bacteria.

Fostamatinib for Persistent/Chronic Adult Immune Thrombocytopenia

“…Plasma half-life ranged from 10.8 to 15.7 h. Overall mean recovery was 99.3%, with 80% of drug recovery in the feces within 96 h, and 19.3% via renal elimination within 72 h [31].…”

“…In a human mass balance study in six healthy adult males who were orally administered 14 C-fostamatinib, R406 was the major drug compound detected in plasma, whereas only low levels of R406 metabolites were found in circulation, suggesting rapid clearance of those metabolites [31]. Plasma half-life ranged from 10.8 to 15.7 h. Overall mean recovery was 99.3%, with 80% of drug recovery in the feces within 96 h, and 19.3% via renal elimination within 72 h [31].…”

“…R406 was initially identified as a relatively specific inhibitor of Fc R-dependent mast cell activation in a cell-based screen for structure activity relationships with primary human mast cells [37]. However, due to the low aqueous solubility of R406, fostamatinib was designed as a methylene-phosphate prodrug that is cleaved to the biologically active R406 by alkaline phosphatases at the apical brush-border membranes of intestinal enterocytes [31,46]. R406 is a potent and selective inhibitor of Syk; it inhibits Syk kinase activity by binding the catalytic domain (K i = 30 nM) and acts as an ATP-competitive inhibitor (IC 50 = 41 nM) [37].…”

“…Fostamatinib is rapidly and completely hydrolyzed to R406 in vitro with alkaline phosphatase and when incubated with human intestinal microsomes [31]. In rats, orally administered fostamatinib was highly bioavailable and showed rapid absorption with dose-proportional systemic exposure (following single fostamatinib dose, 10 or 20 mg/kg: AUC 0-16 h = 10,618 ng · h/ml and 30,650 ng · h/ml, respectively; C max = 2600 ng/ml and 6500 ng/ml, respectively; t 1/2 = 4.2 h) [36].…”

“…is an orally bioavailable prodrug that is metabolized into a biologically active form, R406 (N4-2,2-dimethyl-3-oxo-4H-pyrid [1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine), a selective small molecule inhibitor of Syk (Figure 5) [31,46]. R406 was initially identified as a relatively specific inhibitor of Fc R-dependent mast cell activation in a cell-based screen for structure activity relationships with primary human mast cells [37].…”